Add support for negative prefixes in zIndex plugin

Author: adamwathan

src/plugins/zIndex.js

@@ -1,11 +1,12 @@
 import _ from 'lodash'
+import prefixNegativeModifiers from '../util/prefixNegativeModifiers'
 
 export default function() {
-  return function({ addUtilities, theme, variants }) {
+  return function({ addUtilities, e, theme, variants }) {
     const utilities = _.fromPairs(
       _.map(theme('zIndex'), (value, modifier) => {
         return [
-          `.z-${modifier}`,
+          `.${e(prefixNegativeModifiers('z', modifier))}`,
           {
             'z-index': value,
           },